Plants in use utilizing nuclear units for producing a steam supply utilize nuclear reactor coolant pump heat and decay heat from the nuclear fuel within the reactor to bring a nuclear steam supply system up to a maximum temperature before the reactor is made critical to generate nuclear heat. As the water temperature in the nuclear steam supply system increases, steam is generated which is then boiled off to maintain the nuclear steam supply system at desired pressure and temperature conditions. As steam is boiled off, feed water has to be added to the system to compensate for that lost in the boiled off steam. Feed water added to the system is cold and generally at a low temperature while the metal parts making up the nuclear steam supply system are at their maximum operating temperature. The resulting difference in temperature leads to large temperature gradients across the walls of the feed water piping in the system and of the inlet nozzles in the system with the result that the possibility of thermally induced cracks in the feed water piping walls and nozzles is increased.
Further introduction of cold feed water into a hot nuclear steam supply system reduces the temperature of the water already in the system resulting in a decrease in volume of the water in the system. This decrease in volume of the water already in the system may lead to possible overfilling of the system as the newly heated feed water becomes heated and expands.
It is therefore an object of our invention to provide for a plant incorporating an apparatus and a method of operating the same which will result in a reduction of temperature gradients in a nuclear steam supply system during start up of a plant incorporating such a system and at the same time to provide for a plant which will further minimize overfilling of the system with feed water during start up.